The control of epitaxial processes must be very strict. Using the wrong parameters for an epitaxial process can cause the vertical and horizontal dimensions of the resultant epitaxial structure being outside the specified range for that particular process. This leads to the scrapping of wafers and also to problems with alignment of the wafers in subsequent photolithography processes, which causes a mismatch in components formed on the wafers.
For example, epitaxial process temperature influences sheet resistance, alignment performance and DUF-shift, where DUF is the diffusion under film. The process temperature should therefore have a variance of no more than ±5° C. for any given epitaxial process.
However, the effective temperature of the epitaxial reactor cannot currently be measured inline during the epitaxial process. Therefore, if the temperature should fluctuate from that required for a particular process during the process itself, this would not be apparent until after the entire process has been completed. By this time, other lots of wafers would have been processed in the epitaxial reactor, adding to the wastage.
The present invention has been devised with the foregoing in mind.